Adding a React Component for establishing Context via React Trees

[darthtrevino]

This will allow us to nest the DragDrop engine under an iframe. Note that the iframe window must be injected via context.

see #241
see react-dnd/dnd-core#37

[kesne]

It seems weird to have users put the window object on the react context, I'm not sure what that solves?

[darthtrevino]

The javascript at this point that's assembling the DOM is executed in the context of the outer window global, but the drag-and-drop context within the iframe needs to be constructed with the inner window to establish proper eventing.

[darthtrevino]

We could probably use a prop to inject the inner window instead, but it seems kind of natural to use a context variable to replace a global ¯\(ツ)/¯

[darthtrevino]

I think what will make sense is to do the following checks to determine what window instance to use:

1. Check the window prop on this component. If it has been explicitly set, that should take the highest precedent.
2. Check the window context value. If this is available, it should be used.
3. Use the standard window global.